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second 12 volt battery - 2020 edition

Discussion in 'Gen 3 Prius Main Forum' started by geedub, Apr 27, 2020.

  1. ChapmanF

    ChapmanF Senior Member

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    Dude, you're the one who wants to keep plugging more stuff in your car and wondering how much battery you need. ;)
     
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  2. geedub

    geedub Member

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    Can't I just keep plugging stuff in until something breaks?

    I've been considering a really annoying post (even more than the others), and I think it's time...
     
  3. geedub

    geedub Member

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    The consensus is that it's wise to add an additional 12 volt battery if you want to run a fridge, laptop, etc. I know so little about batteries and electronics I have no business questioning common wisdom, so that's exactly what I'm going to do.

    I know almost nothing, but I think I know that there's a big battery, and a little one that is charged by the big one. The big one is charged by the gas engine whenever it needs it. The little one normally doesn't do much, and the stuff I want to run is powered by the little one. By doing this I'm making the little one work harder, and therefore something bad might happen. What I want to understand is, what is the bad thing that will happen? Will the little battery die quickly? How soon? If I can run my laptop, 60W fan, and a fridge for one year before killing the little battery, is that a bad thing? I think $250/year isn't bad for all of that utility. Will I kill the big battery? I definitely don't want that.

    Kudos to all who modify their cars with additional power, but I do want to understand what the exact concerns are if I simply rely on the stock 12 volt while keeping the car in READY. Are we talking acid fire raining down on me in the the middle of the night, or just replacing the 12 volt more often? Something else? Has anyone tried just using the single 12 volt like I'm talking about?

    Please remember that patience is a virtue. And if you don't have anything nice to say...
     
  4. Terrell

    Terrell Old-Timer

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    With the car on (Ready), you should be fine running up to roughly 83A off the 12V battery. But who wants to be running the car all the time, especially while you go for that hike and the refrigerator needs to run to keep your ice cream from melting during the acid fire raining down? (Oh, you said during the night...) Installing a second 12V battery is not at all hard. See

    With my solar plugged in during the day charging the second 12V, the Dometic will run fine without the car turned on.
     
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  5. ChapmanF

    ChapmanF Senior Member

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    I have. I have never added an additional battery to any Prius of mine, and I've had several hundred watts of inverter plus a hardwired air compressor installed for years, plus a thermoelectric cooler from time to time (which draws considerably higher average current than an Engel or Dometic fridge). I did have to replace my 12 volt battery this past winter at the age of ten years. I don't know how much longer I would have wanted it to last.

    You are on the right track to ask "what are the exact concerns ... while keeping the car in READY". Under those conditions, the correct answer for you is "none." When in READY, the DC/DC converter is supplying the power, not the 12 volt battery, and the capacity of the converter is plenty to cover everything you have so far proposed plugging in and then some, so the battery will not see any load under those conditions. It will be getting charged, in fact.

    You have mentioned adding a 1000 watt inverter, but it doesn't sound as if you intend to be pulling anywhere near that kind of power from it for anything but occasional short periods (InstantPot for 15 minutes to cook a meal?) Do be sure to have the car READY when doing stuff like that. You will be nudging the converter's limits when pulling full load from an inverter like that. You will not be anywhere near the limits when using a laptop, a small fan, a Dometic fridge, etc.

    The calculation changes a bit if the weather gets chilly. There is an electric heat element built into the car's heater that can come on to take the chill off while the engine is warming up, and that animal is about 700 watts. It is turned on automatically by the car without any notice to you, so you just have to remember that can happen. When the heater is pulling those 700 watts, there's a lot less to spare for your toys.

    That electric heater is disabled when you have ECO mode turned on, so you can do that and not have to worry about it.

    That should answer your question as stated, about using your stuff while READY.

    Now Terrell rightly points out that there might be times you don't want to keep the car in READY.

    Those are the times you would probably like to know how long the battery will hold up the loads you care about while not being recharged by the car.

    That's math.

    You have a 45 amphour battery that comes with the car.

    It will be completely drained after supplying 1 amp for 45 hours, or 45 amps for an hour, or 3 amps for 15 hours ... you get the picture. The car itself when off will be drawing a small fraction of an amp, less than a tenth.

    There are some complexities here: in truth, the amphour capacity of a battery is rated at a particular current draw, and isn't exactly the same when supplying current at other rates. You can read long papers about it.

    Usually the way one handles complexities like that in practical situations is not by trying to pin down pages of exact formulas, but by doing the basic back-of-the-envelope calculations and then multiplying or dividing by a decent safety factor, like two.

    So maybe you figure the main thing you want to keep going while your car is OFF is the Dometic, and we don't know what the average draw of the CFX28 is (we know it was given as 0.98 Ah/h for the CFX3 35, but the 28 is from the older, less efficient line). Maybe use the 1.46 Ah/h figure from the comparably-sized Engel.

    Ok, that'll pull 1.46 amps, averaged over time, out of your battery. Don't forget the car itself, should be under a tenth, but call it a tenth. That's 1.56 amps. Divide that into 45 amphours, comes to about 29 hours of run time.

    This is where you compensate for all the fiddly details you don't know by just slamming in a factor of two, and saying "I'm never going to count on more than 14 or 15 hours at a stretch to run my Dometic with the car OFF". That would use about half the battery's capacity, and also require a good several hours with the car READY again for it to be replenished. (Don't forget that part.)

    The Dometic will turn itself off before draining your battery to any point that would inconvenience you, and that might happen right around the 14 or 15 hour mark. Maybe earlier if your battery is old and tired.

    You know an easy way to find out? Turn the car off and see how long the Dometic runs before it turns itself off.

    Doing that one time will absolutely not harm your battery, your car, your cat, or any of your adorable pet alligators and snakes. And then you'll know. If you take that time and lop it in half again, you will have a time that you can be pretty darned confident you'll be able to run the fridge with no trouble at all.

    Then you look at that time and compare it to however long you would like to be able to do that, and if it's less, you look into adding battery capacity, and if it isn't, you call it good.

    Again, keep a jump pack around so you won't be inconvenienced no matter how you miscalculate. In my experience, you will obsess over all the details of fridges and laptops and inverters and the time you need the jump pack will be when you left the map light on. The jump pack doesn't care, it will just make sure you're back in business no matter what the explanation was.

    If you do find yourself seriously draining the battery more than a couple times as you're working things out, keep track of that; repeated deep discharges will take a toll over time. No fiery acid or anything, just an earlier arrival of the day for replacing the battery.

    I probably abuse mine more than most people on PriusChat, what with wiring in weird stuff and some of it, like the compressor, being hardwired, which can sometimes come on while I'm away from the car, and it draws 23 amps. I do have it behind a module that will shut off at a threshold battery voltage, much like the function built into your Dometic.

    I'm pretty sure I work my battery harder than your proposed arrangements will work yours. That should probably leave me feeling guilty for the last one's demise at ten years, but it doesn't, really.
     
    #85 ChapmanF, May 27, 2020
    Last edited: May 27, 2020
  6. geedub

    geedub Member

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    Wow Chapman! That is so helpful I wish I could "like" it 10 times. Maybe I should post while sipping mid-tier California red table wine more often!

    I emailed Dometic with a request for power consumption data and will post it if they respond. In the meantime I found a comment on their CFX28 page by a customer who ran a test which might be helpful.

     
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  7. geedub

    geedub Member

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    Thanks Terrell. Your videos are very well done and make these mods approachable for a layman.

    My needs will be a bit different than a camping situation because I'm really just trying to live a somewhat normal life until the country goes back to "normal" and I can consider permanent housing. Given the hot/humid conditions in Flordia I will run the A/C most nights, mainly to dry out my bedding, clothes and electronics. I also leave camp in my car almost daily to visit family, so I won't often be in a situation where I would have the option of shutting the car off for more than a few hours.
     
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  8. Johnny Cakes

    Johnny Cakes Senior Member

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    I think this creates a mis-impression, even though I'm confident that you didn't mean to.

    This is what people need to know. Adding a second battery doesn't do bupkiss when the car is in READY mode and is just another item that TAKES electricity to charge it up.

    True, a second battery gives more capacity to run things when the Prius is OFF. But given the very small capacity of the Prius battery, I would not run a fridge from the Prius battery in OFF mode, but rather from the second battery which I'd isolate. This would make sure the Prius electronics can be booted with the Prius battery, such that the DC/DC converter can then supply power to start the car. You don't get that many amp hours from a Prius battery anyway.

    Bottom line: for camping in the Prius in READY mode, a second battery adds absolutely nothing.
     
    #88 Johnny Cakes, May 27, 2020
    Last edited: May 27, 2020
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  9. ChapmanF

    ChapmanF Senior Member

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    There still seem to be some persistent notions in circulation that seem to be impervious to any amount of reposting the actual specs.

    One is that the Prius aux battery is crazy small. 45 amphours doesn't qualify as honkin' by any means, but neither is it puny and delicate either. My friend's small sailboat has used batteries of 80 to 100 Ah or so, and motored around lakes on them when the wind quit.

    Also, the Prius battery, because it never needs to start an engine, is built more like a sailboat battery than a car battery, which is advantageous for this kind of use.

    The other notion is that the fridge is a notably heavy load. The specs for some Engel and Dometic models have been reposted here a couple times, and the newest Dometic (35 size) averages under an amp while it is maintaining temp. That's more in the neighborhood of notably light load. The Engel may be around 50% worse, and that might be about where the older CFX28 that geedub is using comes in.

    "Maintaining temp" has to be distinguished from "getting down to temp", which can take the first half-hour to hour if you start with the fridge at ambient temp. During that time it will be running continuously and drawing a handful of amps. That could spend down your battery charge by five amphours or so before it reaches the point of cycling on and off.

    The Gen 1 Prius was originally fitted with a crazy small battery, 23 amphours or so. That led to headaches and Toyota issuing a replacement battery tray and holddown kit to fit a 45 Ah battery. They later kept on improving that battery, so that later on, the same battery they were selling to fit that tray was 54 Ah.
     
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  10. Johnny Cakes

    Johnny Cakes Senior Member

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    Can you talk about depth of discharge and how much of the 45 amp hour capacity you'd be comfortable using on a regular basis?
     
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  11. ChapmanF

    ChapmanF Senior Member

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    My math example in #85 involved applying a factor-of-two comfort factor, and in fact applying it twice, which ends up being around a 25% depth of discharge theoretically, and I would feel absolutely zero discomfort with that.

    And again, the comfort evaluation has to be done by geedub based on how geedub is planning to roll. If it will be mostly operating in READY, then discharges of any depth may not be happening on a regular basis at all.
     
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  12. Minima Domum

    Minima Domum Member

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    General consensus for AGM is never go below 50%, but the higher you keep it the longer it will last. 75% full (or 25% DOD) is super safe and the battery should last for many years.
     
  13. Johnny Cakes

    Johnny Cakes Senior Member

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    That's what I had heard -- so in terms of actual specs, the Prius battery at 45 amp hours, is a max of 22.5 amp hours and a super safe max of 11.25 amp hours.
     
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  14. ChapmanF

    ChapmanF Senior Member

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    Its "actual specs" are its actual specs. Choices about how far you plan on discharging it as you balance lifetime and replacement cost against your needs and weight are choices you make, based on fractions of its actual specs.
     
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  15. geedub

    geedub Member

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    How does that go with a fridge set to shut itself off when the battery reaches 11.9 volts for the HIGH setting, and 11.4 for MEDIUM?
     
  16. Johnny Cakes

    Johnny Cakes Senior Member

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    Probably not actual specs, but here's a chart:
    upload_2020-5-28_18-22-39.png
     

    Attached Files:

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  17. Terrell

    Terrell Old-Timer

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    I learn new things all the time here on PriusChat. I never knew about the electric heater. Is that also on the 2010? I keep the car in ECO mode all the time.
     
  18. Terrell

    Terrell Old-Timer

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    The only reason I mention this is regarding using something like a 1kW inverter with the car in READY mode, which I do while camping to run a small microwave or rice cooker, both of which run for a modest amount of time. Oh, and when the power at home is down, I've run things in the house for seven hours. Yes, the power comes from the DC/DC inverter, the car in in READY mode, and the engine cycles on and off as needed.


    My second 12V battery is isolated from the primary Prius 12V battery, and runs the Dometic, so I don't need the car in READY. It's OFF. The way I have the second 12V battery set up, it's NOT isolated when the car is in READY mode, so that the car charges up both batteries. Also my solar panel charges up both batteries. I have an automatic switch which isolates the two 12V batteries when draining current, and couples the two 12V batteries (parallel) when charging.
     
  19. Terrell

    Terrell Old-Timer

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    That's a good point. I start my trip with the Dometic plugged into 120V to get down to temperature, then load it up with cold food from the refrigerator in the house. So it's not trying to get down to temp from the 12V battery.

    It auto switches to DC when the 120V cord is removed. I have it plugged into the second 12V battery when the frig is in the car, and also plugged into 120V until ready to leave. The second 12V battery is not used until the 120V is unplugged.

    While driving, an auto switch parallels the two 12V batteries, so both batteries are charged by the car, and the Dometic is running off the DC/DC inverter, not the 12V battery. When stopped, the two batteries are auto isolated, so the Dometic runs off the second 12V battery.

    As long as the car is in READY, the Dometic runs off the DC/DC inverter. Only when the car is in OFF does it run off the second 12V battery.

    Add to the story that when parked, a folding 120W solar panel is plugged in, so with the car OFF, the second 12V battery doesn't have to do much work to maintain the Dometic (depending on the sun).

    Specs on the Dometic CFX28 from the manual:
    Rated current:
    12V - 6.5A
    24V - 3.2A
    120V~ 0.65A
    Energy consumption: 61kWh/annum (How do they come up with this? It all depends on so many factors.)

    The rated current draw may be the max when it's running. It cycles on and off, just like any other refrigerator. While off, it draws almost nothing.
     
  20. Terrell

    Terrell Old-Timer

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    From the Manual.................Low Med High
    Switch off voltage at 12V....10.1 11.4 11.8
    Restart voltage.....................11.1 12.2 12.6
     
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